The control of horizontal motion of the locust antenna was studied by behavioural and electrophysiological investigations. Deflection of the flagellum and the pedicel induced by air currents or a needle were used as mechanical stimuli. The forces at this needle during imposed movements were measured with a semiconductor force transducer. The antennal movements were recorded with a capacitative movement transducer, and myograms were recorded simultaneously from the lateral and median scape muscle.
Each scape muscle is innervated by at least three excitatory motoneurones. Reflex movements of the scape-pedicel joint are triggered by several sense organs of the locust, e.g. antennal sensilla. Touching a flagellum of a resting locust normally evokes either an avoidance reflex, which removes the antenna from the stimulus source, or reflex chains, during which the antenna oscillates against the stimulus source. Air-current stimuli induce synchronous phasic-tonic responses in both scape muscles and a stereotyped movement of the antenna (flight-start reaction). This reaction occurs synchronously with the onset of flight muscle activity, and is the beginning of the antennal-positioning reaction that leads to the antennal flight posture. This posture is held constant during steady flight within a range of ±1°. Sudden deflections of the antenna of a flying locust by air-current modulations or by a solid object cause phasic-tonic responses similar to those during the start of flight. Force measurements indicate these responses to be resistance reflexes. Experiments in which one antennal joint was immobilized at a time show the following. (1) Stimulation of only the sensilla of the pedicelflagellum joint triggers transient avoidance reflexes in resting locusts and sustained resistance reflexes in flying ones. (2) Stimulation of the sensilla of the scapepedicel joint alone causes avoidance and resistance reflexes in resting locusts, and resistance reflexes in flying ones.